The present invention relates to a power supply circuit, display device and electronic instrument.
In recent years, it has strongly been desired in the field of portable electronic instruments such as portable telephone, and pager to prolong display time without exchange of battery in addition to reduction of size and weight. It is thus severely required that a display device build in a portable electronic instrument less consumes the power.
The inventor had widely studied liquid crystal display devices which are of one of various display types, in view of reduction of power consumption.
As a result, it has been found that the conventional liquid crystal display devices had their power supply circuit for supplying a power voltage, which itself consumed very large amount of power. The power supply circuit required about ⅓ of the power to be consumed in the liquid crystal display device.
To overcome such a problem as described, an object of the present invention is to reduce the power consumption of the power supply circuit itself and thus the power consumption in display devices and electronic instruments that use such a power supply circuit.
To this end, the present invention provides a power supply circuit for converting a voltage and for supplying the converted voltage as a power supply voltage, the power supply circuit comprising:
at least one charge pump circuit which includes a first capacitor, a second capacitor, a first switching means for charging the first capacitor based on a given voltage, and a second switching means for transferring the charge in the first capacitor to the second capacitor; and
a switching signal generation circuit for generating a plurality of switching signals which control the first and second switching means;
wherein the first switching means includes a plurality of switching elements, one ends of the switching elements being electrically connected to different potentials, and the other ends thereof being electrically connected to at least one end of the first capacitor; and
wherein the switching signal generation circuit receives at least one given first control signal for controlling at least one of the boosting (step-up) ratio and the deboosting (step-down) ratio, and then generates the switching signals for controlling ON and OFF-states of one of the switching elements specified by the first control signal, and for turning off at least one other switching element.
According to this aspect of the present invention, for example, the first switching means may comprise first, second and third switching elements respectively connected at one end to first, second and third potentials. If the first control signal is used to set a first boosting (or deboosting) ratio, the first switching element is ON-OFF controlled while the second and third switching elements are turned OFF. Thus, the first capacitor will be charged based on the first potential. On the other hand, if the first control signal is used to set a second boosting ratio, the second switching element is ON-OFF controlled while the first and third switching elements are turned OFF. Thus, the first capacitor will be charged based on a second potential. As a result, there can be provided a converted voltage different from that obtained by using the first potential. Similarly, if the first control signal is used to set a third boosting ratio, the first capacitor will be charged based on a third potential to provide a converted voltage different from those obtained by using the first and second potentials. According to the present invention, thus, the boosting or deboosting ratio can variably be controlled by the first control signal. Furthermore, there is an advantage that the boosting and deboosting ratio can variably be controlled while effectively preventing the output impedance of the power supply circuit from being increased associated with the addition of any new switching element or the overall circuit from being increased in scale.
The plurality of switching elements may be connected to at least one end of the first capacitor. Further, a structure that the opposite ends of the first capacitor are connected to a plurality of switching elements is also within the scope of the present invention.
The switching signal generation circuit may comprises: a circuit for generating a basic switching signal; a decoder for decoding the first control signal; and an output circuit for receiving the output of the decoder and the basic switching signal to output a switching signal generated based on the basic switching signal toward one of the switching elements to be ON-OFF controlled and to output a switching signal fixed at a given potential toward at least one other switching element not to be ON-OFF controlled. In this way, a switching signal for controlling ON and OFF of one switching element and for tuning off the other switching elements can simply be generated. In addition, various types of switching signals can be generated merely by changing the wiring or other section of the decoder.
The output circuit may include a level shifter for converting the amplitude of the basic switching signal on the basis of a reference potential as well as a charge pump potential from the charge pump circuit. Thus, the system can generate a switching signal that has an amplitude required to ON-OFF control the switching elements.
The switching signal generation circuit may receive a reference potential and a charge pump potential of the charge pump circuit for setting the potentials of switching signals during the OFF-state period outputted toward switching transistors included in the first and second switching means at one of the reference potential and the charge pump potential both of which are supplied to the source of the switching transistors. In this way, the switching transistor can properly be turned off during the OFF-state period of the switching signal to prevent the power from being consumed unnecessarily.
The present invention further provides a display device comprising: the aforementioned power supply circuit; a drive circuit for outputting scan and data signals based on the power supply voltage from the power supply circuit; and a panel having scan lines into which the scan signals are inputted, data lines into which the data signals are inputted, and a display element driven by the scan and data lines; wherein at least one of the boosting and deboosting ratios is varied by varying the first control signal according to the duty ratio in the panel. The unnecessary power consumption can effectively be reduced since the boosting and deboosting ratios can be controlled according to the duty ratio.
In the display device of the present invention, a given second signal may be used to select K scan lines among N scan lines and to unselect (N-K) scan lines for performing a partial display; and at the partial display, the first control signal may be varied depending on the number of selected scan lines to vary at least one of the boosting and deboosting ratios. In this way, the partial display can be made to divide a screen into a display area and a non-display area while effectively preventing any unnecessary power consumption.
The present invention further provides an electronic instrument comprising the aforementioned display device and a central control means for processing for setting the first and second control signals. For example, the first and second control signals can be set through a software on the central control means such as CPU and MPU in the electronic instrument.
The present invention further provides a power supply circuit for converting a voltage and for supplying the converted voltage as a power supply voltage, the power supply circuit comprising:
at least one charge pump circuit which includes a first capacitor, a second capacitor, a first switching means for charging the first capacitor based on a given voltage, and a second switching means for transferring the charge in the first capacitor to the second capacitor; and
a switching signal generation circuit for generating a plurality of switching signals which control the first and second switching means;
wherein the switching signal generation circuit receives a reference potential and a charge pump potential of the charge pump circuit for setting the potentials of switching signals during the OFF-state period outputted toward switching transistors included in the first and second switching means at one of the reference potential and the charge pump potential both of which are supplied to the source of the switching transistors.
The potential of the switching signal inputted into the switching transistor during the OFF-state period can be equal to the reference potential or the charge pump potential supplied to the source of that switching transistor. Thus, the switching transistor can properly be turned off. Since the amplitude of the switching signal can be reduced, any unnecessary power consumption can effectively be prevented.
The switching signal generation circuit may set the potentials of the switching signals during the OFF-state period based on a plurality of charge pump potentials from a plurality of charge pump circuits. Thus, when a final converted voltage is obtained by the plural charge pump circuits, the potentials generated by these charge pump circuits can effectively be utilized.
The switching signal generation circuit may comprise: a circuit for generating a basic switching signal; and a level shifter for converting the amplitude of the basic switching signal based on the reference and charge pump potentials. When such a level shifter is used, a switching signal which is used to ON-OFF control the switching transistor and to equalize the potential of the switching signal during the OFF-state period with the source supply potential can simply be generated.
The present invention further provides a display device comprising: the aforementioned power supply circuit; a drive circuit for outputting scan and data signals based on the power supply voltage from the power supply circuit; and a panel having scan lines into which the scan signals are inputted, data lines into which the data signals are inputted, and display elements driven by the scan and data lines. The display device can extremely be reduced in power consumption.
The present invention further provides an electronic instrument comprising the aforementioned display device, and a central control means for processing for controlling the display device. Thus, any electronic instrument such as portable telephone, printer, personal computer, pager, and projector can be reduced in power consumption and improved in battery service life.